1. Field of the Invention
This invention relates to a catalyst suitable for the selective production of para dialkyl substituted benzenes and to a process for converting specified charge stocks to a high yield of para dialkyl substituted benzenes utilizing such catalyst.
2. Description of the Prior Art
The disproportionation of aromatic hydrocarbons in the presence of zeolite catalysts has been described by Grandio et al. in the Oil and Gas Journal, Vol. 69, No. 48 (1971). U.S. Pat. Nos. 3,126,422; 3,413,374; 3,598,878; 3,598,879 and 3,607,961 show vapor-phase disproportionation of toluene over various catalysts.
In these prior art processes, the xylene product produced has the equilibrium composition of approximately 24 percent para, 54 percent meta and 22 percent ortho.
In addition to the above patents, other related prior art includes U.S. Pat. No. 2,904,607 which refers to alkylation of aromatic hydrocarbons with an olefin in the presence of a crystalline metallic aluminosilicate having uniform pore openings of about 6 to 15 Angstrom units. U.S. Pat. No. 3,251,897 describes alkylation of aromatic hydrocarbons in the presence of X- or Y-type crystalline aluminosilicate zeolites, specifically such type zeolites wherein the cation is rare earth and/or hydrogen. U.S. Pat. Nos. 3,751,504 and 3,751,506 describes vapor phase alkylation of aromatic hydrocarbons with olefins, e.g. benzene with ethylene, in the presence of a ZSM-5 type zeolite catalyst.
The alkylation of toluene with methanol in the presence of a cation exchanged zeolite Y has been described by Yashima et al. in the Journal of Catalysis 16, 273-280 (1970). The workers reported selective production of paraxylene over the approximate temperature range of 200.degree. to 275.degree. C., with the maximum yield of para-xylene in the mixture of xylenes, i.e. about 50 percent of the xylene product mixture being observed at 225.degree. C. Higher temperatures were reported to result in an increase in the yield of meta-xylene and a decrease in the production of para and ortho-xylenes. U.S. Pat. No. 3,965,210 describes alkylation of toluene with methanol in the presence of a crystalline aluminosilicate zeolite, such as ZSM-5, which has been modified by contact with a polymer made up of meta-carborane units connected by siloxane units to selectively yield paraxylene. These latter catalysts have, however, suffered from the serious deficiency of loss of selectivity upon air regeneration. This is attributable to breakage of carbon-silicon bonds upon exposure to the high temperature of regeneration giving rise to isolated clusters of silica on the zeolite surface rather than the extensive surface coverage afforded by the technique described herein.
U.S. Pat. No. 2,722,504 describes a catalyst of an activated oxide such as silica gel having a thin layer of a silicone polymer deposited thereon to increase the organophilic character of the contact surface and, as such, seeks to avoid silica deposition.
Crystalline aluminosilicate zeolites, modified by reaction with an organic substituted silane, have been described in U.S. Pat. Nos. 3,682,996 and in 3,698,157. The former of these patents describes, as novel compositions of matter, crystalline aluminosilicate esters made by reacting a crystalline aluminosilicate having an available hydrogen atom with an organic silane having a SiH group. The resulting compositions were disclosed as being catalysts useful for hydrocarbon processes, particularly hydrocracking. In the latter of the above patents, the use of ZSM-5 type crystalline aluminosilicate zeolites modified by treatment with an organic-radical substituted silane are described, together with the use of such modified zeolites in chromatographic separation of the compounds in a C.sub.8 aromatic feed stock.
While the above-noted prior art is considered of interest in connection with the subject matter of the present invention, the conversion process described herein utilizing a catalyst of particularly defined sorption properties comprising a crystalline aluminosilicate zeolite characterized by an alpha value of between 2 and about 5000 modified by a coating of silica derived from a silicone, has not, insofar as is known, been heretofore described.
Of the xylene isomers, i.e. ortho, meta and para-xylene, meta-xylene is the least desired product, with ortho and paraxylene being the more desired products. Para-xylene is of particular value being useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers, such as "Dacron". Mixtures of xylene isomers, either alone or in further admixture with ethylbenzene, generally containing a concentration of about 24 weight percent para-xylene in the equilibrium mixture, have previously been separated by expensive superfractionation and multistage refrigeration steps. Such process, as will be realized, has involved high operation costs and has a limited yield.